There are three main UPS types and each is intended to keep a system protected from blackouts, brownouts and catastrophic events.

1. Standby

A Standby or “Offline” UPS system’s load is powered directly by the input power. When the voltage becomes too high or too low, the UPS automatically switches to battery backup mode.

There is a transfer time that occurs to go between normal power and battery power. The switchover can take as long as 25 milliseconds (ms) depending on how long it takes the standby UPS to detect the lost utility voltage. The UPS is designed to power certain equipment such as personal computers.

2. Line Interactive

A Line Interactive UPS is similar to a Standby UPS but with the addition of a multi-tap variable voltage autotransformer that provides built-in voltage regulation – commonly called a buck/boost capability. The special type of transformer can add or subtract powered coils of wire, thereby increasing or decreasing the magnetic field and the output voltage of the transformer.

This type of UPS is able to tolerate continuous under voltage brownouts and overvoltage surges without using the batteries, which helps to preserve battery life. When the voltage is too high or too low for the buck/boost capability, the UPS will automatically transfer to battery power. There is a transfer time that occurs between normal power and battery power, however, unlike a standby UPS, the transfer time is very quick and should occur in less than 5 ms.

3. Online

An Online UPS provides a constant source of electrical power from the battery, while the batteries are being recharged from the incoming AC power. It uses a “double conversion” method of accepting AC input, rectifying to DC for passing through the rechargeable battery (or battery strings), then inverting back to the necessary AC voltage for powering the protected equipment.

With Online UPS systems, the batteries are always connected to the inverter so there is zero transfer time when an outage occurs. When power loss occurs, the rectifier simply drops out of the circuit and the batteries keep the power steady and unchanged. When power is restored, the rectifier resumes carrying most of the load and begins recharging the batteries.